Once Upon A Time... by Thomas

Going out of the finale, I imagine a fair amount of people are disappointed that it is never stated what the Island actually IS... but, given the information from the finale, I think we can surmise exactly what the Light (or the electromagnetism) is.

Desmond first came in contact with it after detonating the hatch, and he started seeing visions of the future. The second time he was hooked up to electromagnets by Widmore and he travelled into the "afterlife" (or the in between). During the finale, he displayed a remarkable knowledge of it.

I think the Well is nothing less than the gates to heaven, or whatever you like to call where our characters are heading. Jacob (or Jack, or Hurley) fulfills the part of the gate's protector. His brother was an unfortunate side-effect of his predecessor's (Mother) blessing (couldn't die) and the gate's destructive effect.

No living person can go through it, although Desmond could survive being in the proximity of it. This also raises the interesting question of what happened when Desmond removed the cork (it was literally a cork!). Hell itself seemed to rise up, except it could also be explained as volcanic activity (hinted at before in the series). Just a matter of looking at it from the perspective of science or faith, which I'm sure is why they kept the reveals deliberately vague.